Xenon lamps, which are used in light-fastness testing apparatus, in general have a lifetime of several hundred hours, the light emitted being strong when the lamp is new, but the intensity decreasing gradually with the passage of time. The intensity of the light, however, does not decrease uniformly over the whole of the spectrum of radiation emitted by the lamp, i.e., in the ultraviolet region (wavelengths of 300 to 400 nm), visible light region (400 to 700nm) and infrared region (700 to 1200 nm). Therefore, in conducting light-fastness tests, the results obtained using a new lamp are often not in agreement with the results obtained using an old lamp even for the same samples.